The goals of this project are to seek to further our understanding of organ of Corti function by studying its physiological responses to sound and electrical stimulation with selected manipulations. Inactivation or electrical activation of the efferent neural system will be used to alter "normal" cochlear function. Cochlear responses will be measured as gross potentials and ear canal otoacoustic emissions. The role of the efferent neural system in cochlear processing remains unknown. Aim I will examine alterations in cochlear responses as a consequences of eliminating the efferent neural system, or over-stimulating it. These manipulations will focus on the efferents role in the improvement of signal detection in noise. The outer hair cell is crucial to normal cochlear function. The effects on cochlear responses caused by activation of the efferents is mediated through the outer hair cell. Of interest is the cochlea's response to conditions in which the efferents are inactivated or stimulated for extended periods of time. Aim II will examine the effects of electrical activation of the efferent system on the electrically evoked otoacoustic emission (EEOAE) recorded in the ear canal. The characterization of the EEOAE is the primary focus of project 6 and also studied in project 2. The relatively new method of stimulating the cochlea by electrically stimulating the round window provides an exciting way to study organ of Corti function. The outer hair cells are thought to be the source of the EEOAE and reflects the reverse transduction process. The outer hair cells in the base of the cochlea are operating at a maximum gain point in terms of the mechanical properties of the outer hair cells and the reverse transduction process. The innervation of the basal outer hair cells by the medial efferent system provides a means of altering the gain point of the outer hair cells. Of particular interest is the possibility of differentiating and furthering our understanding of the "slow" efferent effect. In contrast to the "fast" efferent effect, the slow effect appears to be related to protective phenomenon that reduces hearing loss as a result of acoustic trauma. A better understanding of this phenomenon has clear potential benefit to humans exposed to loud sounds.